Cathodic protection systems are widely used to protect for example buried metal structures or steel in reinforced concrete. Two types of systems are usually employed; those using gavanic anodes, and those using impressed current anodes. In either case, reference electrodes are widely used in cathodic protection systems to maintain and monitor the system at precise protective potential levels. Reference electrodes are often connected to electronic control systems which monitor and maintain cathodic protection systems on a continuing basis. Readings obtained from a reference electrode are, however, only as reliable as the reference electrode itself.
Underground or underwater reference electrodes of the copper-copper sulfate or silver-silver chloride type have been developed for use with cathodic protection systems and are widely used. An example of such reference electrodes or cells are those sold by Harco Corporation of Medina, Ohio under the trademark PERMACELL.RTM.. Such underground cells consist essentially of a relatively elongated plastic tube equipped with a special ion trap or wooden plug at the end of the cell to prevent contamination of the cell by the surrounding electrolyte. The tube contains a plaster/copper sulfate mix. The cell is then prepackaged in a special backfill to ensure both low contact resistance and long term stability.
Since the materials react differently, there are numerous problems in the fabrication, storage or shelf life, and long-term life. All the components must interact in order for the cell to last. If it freezes, it no longer will operate. If it dries out, it stops working. Fabricating the cell is labor intense and requires some type of controlled environment. The assembly procedure must be strictly adhered to throughout fabrication. Cells cannot be recharged if anything should fail, or, if one thinks it has failed, it cannot be verified.
In a typical system one or more of the reference cells may be installed at various locations in, around or beneath a structure being protected and connected either in parallel to provide an average potential reading or into a rotary switch, for example, enabling the operator to select any one of the reference cells as a signal source for the automatic system. Such cells have a design life of approximately fifteen years.
To replace such a cell in an inaccessible location can be much more expensive than the cell itself. This is especially true if the cell or electrode is buried under the structure of concern or is utilized in a reinforced concrete structure such as bridge decks, piers, pavement, garages, etc., of both the poured-in-place and precast type.
Also, such cells usually only have a single lead and thus it is difficult if not impossible to test the cell for accuracy or stability. Also, the environment of a structure being protected can become quite dry and in such dry environments the resistance between the cell and the surrounding electrolyte increases. Moreover, in any half cell or electrode its stability is fundamentally dependent on the metal ion concentration around it. Such ions tend to leach away and this is particularly true in extremely dry environments.